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Thread: Eric Barbour's 6BM8 One-Tube Reverb Imagined...

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    Eric Barbour's 6BM8 One-Tube Reverb Imagined...

    Since I can't find the schematic for Eric Barbour's famed one-tube 6BM8 reverb for Fender amps, I decided to take his example (Fig. 2 from this article: Tube-Town Germany - Hot Stuff Cool Sounds) and flip it around backwards to form a complete one-tube reverb circuit.

    BUT, I don't know what I'm doing (been a while since I did this sorta thing). So, I need your help double-checking the schematic, especially since I don't understand grid bias, etc.

    The original schem has a 180K resistor off of B+ which eventually feeds the control grid on pin 3 of the 6BM8. I plan on using the 5F1 champ's first preamp stage, which has a 100K resistor there instead.

    Here's Eric's original schematic:

    sv6bm8_02.gif

    And here's my imagining of his reverb circuit that needs double-checking:

    6bm8singletubereverb1a.png

    Thanks in advance for any help you can provide!
    Last edited by dchang0; 11-15-2010 at 09:20 PM.

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    I don't know what reverb tank you intend to use, but you should consider the impedance of it. Likewise, when you flip around the stages of the reverb circuit like that, be mindful of the impedance.
    Last edited by überfuzz; 11-15-2010 at 08:53 AM.
    In this forum everyone is entitled to my opinion.

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    It'll be the Accutronics 8EB2C1B, with 800 ohm input and 2575 ohm output. It's a shorty tank (this is going into a 5F1 Champ cabinet).

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    Ehh, did you say you wanted to mod a 5F1..? Did you ever consider leaving it as it is and get a fender amp with a stock on-board reverb. Off topic, but still...

    Right. Why don't go for a Fender reverb circuit. Most of the on-board reverb circuits by fender are the same. You'll have a fender amp, with a fender reverb. Yey!!! I think you'll be able to use you're tank to. You might have to adjust some values to make if work nicely.
    In this forum everyone is entitled to my opinion.

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    Heh, heh, heh--'cause I like sitting up till 2AM reading about tube amp design?

    LOL. All kidding aside, there is a real reason for wanting a 6BM8 one-tube reverb. I'm using a real 5F1 cabinet and chassis, which are tiny (and into which I can only fit one extra 9-pin tube). So copying any true Fender reverb circuit is out, as they tend to use multiple tubes for their reverb.

    As for why a 5F1--space and especially wattage considerations dictate the small size. I am actually selling off my only-slightly-larger Princeton Reverb build (which has a great reverb) and keeping the 5F1 build. The key is: I don't PO the gf (space) or the neighbors (wattage)...

    And plus it actually IS fun to go back and re-learn about tube circuits. It was almost 20 years ago when I studied this stuff (which is why I need help spotting any errors...)

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    My 2 cents: You probably need a grid leak on the triode, 330k to 1M from grid to ground. The reverb output says reverb send twice, just a syntax error there.

    Don't tie the reverb send ground to chassis ground as depicted, just run the 2 transformer leads straight to the tank isolated. Ground to chassis only the tank return ground.

    .22 return cap seems too big. 0.022 more like it. Even smaller will do, my last one was in fact 1nF

    2M return pot seems way too much as well, my last project used 250k because you want the return gain lower than send, it's very noisy on the single coil pickup at that tank.

    Hope these ideais help somehow.
    "Tell them I said something." - Pancho Villa's last words
    For Portuguese speakers: Compreenda seu Amplificador Valvulado

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    +1 on what JMAF says, although the return cap probably depends upon the how much signal you have filtered at the input.
    You'll also need to look at the amount of drive you need for the pentode section of the 6BM8. It's not a small bottle EL84 regardless of what you may read; it's more like a 6AQ5 or small bottle 6V6. Secondly, the tank that you have isn't really optimal for the transformer output. You might have better luck by making the pentode a triode which still offers plenty of output and makes the circuit less picky about impedance.
    You might also look into using a transistor recovery stage-- a JFet/Mosfet cascode circuit works really well for this, probably better than a single tube.
    Merlin's site The Valve Wizardhas some interesting circuits for medium impedance tanks as well.

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    Thanks, guys!

    Okay, I updated the schem with jmaf's suggestions (all good ones). It is currently at Rev. 1.0a.

    If it helps to figure out the proper return cap value, the input stage is the very common, standard 12AX7 triode first gain stage on the 5F1, virtually identical to the triode stage of the 6BM8 as shown (100K plate resistor instead of 180K, 22uF cathode cap instead of 47uF, all other components the same values).

    @BiBi--it's funny, I happened to be reading The Valve Wizard's article on reverb drivers last night at 1AM! Eye opening for sure... I'll have to go re-read it in detail to get a grasp on the math involved.

    Using a triode for the driver section is definitely an option. Others have said that a 12AX7 is too weak to drive a lush reverb circuit. I chose the 6BM8 mainly because of all the good results people have been getting with them (surprisingly few schems out there, though). Plus, to some degree, figuring out the 6BM8 is a learning experience for me, as there are plenty of "plug-n-play" one-tube-12AX7 reverb circuits out there that I could simply copy without thinking. (I still may copy them simply because it appears that the number of components can be reduced by one or two, saving space.)

    As for the lack of an optimal tank, I happen to have on hand a Heyboer reverb transformer (25000 ohms red-blue, 8 ohms green-black) that's a drop-in replacement for the Fender reverb driver. If need be, I could buy another, optimal transformer and sell this one.

    I haven't yet ordered any reverb tank and so could easily choose a better one. HOWEVER, it doesn't appear that tubesandmore.com has anything higher than the 800 ohm input model in the Type 8 Accutronics, though I believe Accutronics does make much higher impedance models. I have no idea where to buy those.
    Last edited by dchang0; 11-15-2010 at 07:36 PM.

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    you want a lower impedance tank with transformer coupling. I think the "A" model is the most common with 10ohm input.
    That said, your transformer is for a typical Fender style circuit with a 12AU7, thus the high primary impedance. I believe the single ended 6BM8 pentode OT would be more in the 6-8k range.
    I personally like the Ampeg style of capacitor coupling but then I'm a cheapskate!

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    Oops. I thought you were referring to the later paragraph in The Valve Wizard's article where he says "Cheap current source drivers:
    Low impedance reverb tanks seem to be persisting in valve amp design, which makes little sense since valves are not well suited to driving low impedance loads. By taking advantage of high-impedance tanks (usually intended for op-amp drivers) we can build much simpler valve drivers." The example circuit he gives with paralleled ECC82s is nice in that there's no reverb transformer, but I don't have room for two tubes. Maybe if I could pull it off with the pentode side of the 6BM8....

    He does suggest low-impedance tanks in the paragraphs before that (where, like you pointed out, he suggests using a triode instead of a pentode).

    Which do you think would be better-sounding? I'm at that fork in the road where I could go either way. I LOVE lush reverb (got to, if I'm crazy enough to try to stuff an entire tube reverb circuit into the tiny 5F1) and frequently crank it up to surf levels (though I don't play surf guitar).

    The goal is to have the reverb be as true a representation, tonally, as the original signal.


    The Ampeg method is definitely good for saving both $$$ and physical space. I might try that, though most who have used it say that it's nowhere near the Fender surf sound...


    P.S. All this raises a new question: Would it be wiser to use the triode side of the 6BM8 to drive a low-impedance tank, then use the pentode side as the recovery section? Would I be able to get the surf sound outta that, do you think? The Valve Wizard warns against using the 12AX7 and suggests the 12AT7 or 12AU7; I'd heard that the triode side of the 6BM8 is a lot like the 12AX7, so this might not be such a great idea...
    Last edited by dchang0; 11-15-2010 at 08:04 PM.

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    Well first off the Fender surf sound I believe is from the stand alone unit. I haven't heard any of the amps that sound that crazy! I think when you put the springs into a combo amp bad things can happen. You always have to make compromises such as more filtering that you may not have to in the stand alone. As well, the stand alone unit uses three tubes I believe.
    You could do something in that vein with a 6BM8 and a second tube pretty easily. Just don't give short shrift to the recovery section as that's just as important and maybe somewhat more difficult. I would think of say a 12AX7 as the input, volume control into the triode-connected pentode section of the 6BM8 capacitor coupled into a high impedance tank like the 8F.... Then use the triode section of the 6BM8 as the first section of recovery through a tone control, volume and into the second side of the 12AX7.
    As you'll need a "dry" signal as well, try a source follower transistor setup like a mosfet irf820.
    That's just my 2 cents. I have done something similar with a EL84, 12AT7 and a 12AX7. I could get a really wide arrange of sounds especially with the tone control.
    The unit is now being used in a music studio.

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    LOL--crazy is right! Well, there are definitely a ton of constraints here that will force a serious compromise on the sound, so while "aiming for the surf sound" is cool, actually getting the surf sound is highly unlikely.

    I like your idea, though there just isn't enough physical room for the circuitry. Here's what I'm thinking of doing:

    Guitar input -> 1st 1/2 12AX7 -> split off reverb from dry -> reinsert reverb -> volume pot -> 2nd 1/2 12AX7 -> 6V6.

    Reverb: [-> pentode of 6BM8 -> tranny -> tank -> triode of 6BM8 -^]

    I like your idea of doing a straight capacitor coupling into the tank:

    Reverb: [-> pentode of 6BM8 in triode wiring -> tank -> triode of 6BM8 -^]


    I only have space for the one volume pot on top plus stealing one of the instrument jacks to install the reverb pot. (The problem is that there's only 6.5" of control panel on top, and the chassis is predrilled in the standard 5F1 layout, so there's no room to fit another pot.)
    Last edited by dchang0; 11-15-2010 at 08:54 PM.

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    Oh I should actually read what you wrote at the beginning-- I had no idea you were trying to fit this into something so small. The problem is that when things are packed in really tight, spring transducers tend to feedback and fighting that is absolutely no fun. If I had to build a reverb in something like that I'd probably go for the Belton digi-log. You can also drive these with a single tube and they sound pretty darn good I think. Maybe you should save your parts for a stand alone unit-- the general consensus is that reverb with SE amps doesn't go so well as you generally want a really clean sound to go with reverb.

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    No biggie. Really, I shouldn't be talking too much about the 5F1 idea because this thread is meant to help the DIY community regarding the hard-to-find Eric Barbour 6BM8 mod to Fender amps.

    I am planning on starting another thread with the 6BM8 circuit actually inserted into the 5F1 after I figure out a pretty good (generic) approximation of the Eric Barbour circuit. So my own preferences (surf sound) and constraints (tiny chassis and cabinet) don't really matter much here.

    I'll go check out the Digi-log. One BIG question, though--does it replicate the spring "slap-back" that happens when you palm-mute the strings and hit 'em hard? (Best example that comes to mind is in John Mayer's Slow Dancing in a Burning Room.) If it doesn't do that, then I have to stick with spring tanks.


    And yeah, you're right--reverb with SE doesn't sound all that great, but again, I've got those crazy constraints on size and loudness... I'd love to keep my Princeton Reverb clone (sounds amazing), but it doesn't fit into our modular shelving like a Champ does.

    Thanks for all the help, BiBi!

    Oh, to anybody who happens across this thread, now that the circuit is beginning to deviate too much from what is likely to be the Eric Barbour reverb, I'll start posting newer schematics separately from Rev. 1.0a.

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    To anyone following us down the path of using the triode-strapped pentode of the 6BM8, here is a link to hand-collected curves of that mode of operation:

    http://www.tubes.mynetcologne.de/roe..._as_triode.pdf

    Okay guys, I'm just thinking out loud here.

    From The Valve Wizard:

    "Accutronics quote the saturation current of their cores as 3.5Amps per turn (rms),...

    The voltage required to achieve the Nominal Drive Current can be found from Ohm's law:
    V = IZ
    Where Z is the impedance of the coil, which is normally quoted at 1kHz.

    For example, if a type A tank is rated as 8 ohms at 1kHz and has 124 turns;
    I(nominal) = Amps per turn / number of turns.
    I(nominal) = 3.5 / 124
    = 28mA"

    Now, the curves on the triode-strapped pentode 6BM8 indicate that it can easily provide around 40mA to 55mA in the middle of its curves. That's almost double the 28mA needed for the 8A-model 8-ohm input reverb tank. But Rod Elliot notes "After getting a new 8 ohm tank for some experiments and to take a few measurements, it turns out that the coil can be driven somewhat harder than claimed. I was able to drive the 8 ohm coil to 250mA at 1kHz before saturation (almost 10 times the current claimed). The saturation current remains roughly the same at all frequencies from around 300Hz and up, and at 1kHz the voltage was measured at 2V RMS."

    So while the 55mA output of the triode-strapped pentode 6BM8 might be more than the rated saturation current, it's not enough to truly saturate it.

    Looks like I'll HAVE to buy the 8-ohm tank, as the 8B-model 160-ohm's nominal drive current is a mere 6.5mA, which can easily be driven to 10x = 65mA by the 6BM8.
    -----

    Looking at the chart, It would probably be better off choosing a load curve closer to 40mA than 55mA. I selected 44mA to correspond with a 6.2K plate resistor.

    -----

    Again, looking at the chart, I selected a middle-of-the-road bias for the highest headroom: -14V. That's about 20.5mA quiescent current, just under the 28mA needed for the 8A tank.

    -14V / 20.5mA = 682.9 ohm ~= 680 ohm for the cathode bias resistor.

    Power rating?

    P = RI^2 = 680 ohm * (0.0205A)^2 = 0.285W < 0.5W. Let's go with 1W to be safe.

    This does raise the question: why did Eric Barbour use a 10W (750 ohm) resistor? Is my 1W rating going to be too low?

    -----

    Going back to power rating for the plate resistor:

    P = RI^2 = 6.2K * (20.5mA)^2 = 2.605W. Crap. Gotta step up to 5W wirewound, and because of this, 6.2K is out. 6.8K is available instead, lowering the load curve to 40.4mA.

    Gotta redo the calculations on the new load line. Quiescent current is now 20mA at -14V bias.

    Cathode bias resistor = -14V / 20mA = 700 ohm =~ 680 ohm. No change, then.
    Last edited by dchang0; 11-15-2010 at 11:50 PM.

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    Continued thinking-out-loud:

    Cathode cap:

    Ck = 1/(2*pi*Rk*f-half-boost-k) = 1/(2*pi*0.68k*5k) = 1/21.352 = 0.0468 ~= 47uF

    Voltage rating: 25V is fine.

    For the strapping resistor, let's choose 1W to be safe.

    -----

    Now for the coupling section to the reverb send... I'll start with the default values from The Valve Wizard's paralleled ECC82 transformerless reverb example.

    Here's the new schematic:

    6bm8singletubereverb2.png

    Please let me know if you see any errors in my calculations or shematic--thanks!
    Last edited by dchang0; 11-16-2010 at 12:12 AM.

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    Ra = 6.8K, ra = 1.875K.

    Zout = 6.8 * 1.875 / ( 6.8 + 1.875) = 12.75 / 8.675 = 1.47K

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    Oops. Turns out that 3W metal oxide or 5W wirewound choices are 5K or 10K for Ra, not 6.8K. 5K has a better load line. At -14V bias, quiescent current is 25mA, just under 28mA for the 8A tank.

    -14V/25mA = 560 ohm for cathode bias resistor. We have exactly that in a 1W metal oxide.

    Cathode cap:

    Ck = 1/(2*pi*Rk*f-half-boost-k) = 1/(2*pi*0.56k*5k) = 1/17.584 = 0.0568 ~= 50uF.

    Ra = 5K, ra = 2K.

    Zout = 5 * 2 / (5 + 2) = 10/7 = 1.43K

    Max power, load should be ra=2K doubled = 4K.

    RI should be 20K.

    13mA at 12Vp-p at the bias point of -14V on the Ra=5K load line.
    Last edited by dchang0; 11-17-2010 at 09:36 AM.

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    While you are certainly better at math than I am, methinks you aren't getting the impedance issue correct. Driving such a low impedance (8ohm) directly from the plate of the tube ain't gonna work well! Reread what Valve Wizard says about using an output pentode-- you'll want to use the higher impedance tank like the 8F or you would need an appropriate output transformer!

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    Yup, you're correct. I spent all last night struggling with the impedance matching. Couldn't figure out the formulas to calculate the proper load. Couldn't even figure out which formulas to use! I'm stuck right now--if you can help walk me through the calculations, that would be awesome!

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    Well I would love to walk through calculations but I tend to always get stuck in the mud!
    I think the first thing is to find the optimum load for the tube right? So we know that's 8k in SE pentode. Usually I would think up that a bit in triode, say 10k. (Now you can see I'm not very scientific!).
    Since the load is resistive not reactive, you'll want to bias it more like a preamp than a power amp connected to a transformer which I think you got already.
    So what I do is look at the Philips datasheet, which states 27mA plate current in SE at 270V. From your datasheet, 275V and 27mA puts the bias around 18-20 which gives you a cathode resistor of 680.

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    Hey, any help you can give me is very much appreciated. Even if it's as simple as pointing me to the right article or book or online tutorial...

    I sorta glossed over the load calculations above, but the optimum load for this triode-strapped pentode 6BM8 as shown is 4K. That's as shown, with all the calculated resistor and capacitor values. I used the formulas shown in The Valve Wizard's online article on reverb drivers, and came up with the first bridging resistor (RI = 20K).

    But now, I'm trying to figure out the proper value of the 820K resistor (formerly 1100K) AND the correct impedance of the reverb pan. This requires a bit of RC circuit theory to go back and "reverse-calculate," so I'm having to go back through the formulas and try to figure out which one applies to this topology.

    Oh, I forgot to post the latest schematic. This is 2.0a, based on the 5K/5W Ra resistor. Barbour went for the 10K impedance via the transformer, resulting in a lower load line. I went up, towards a hotter load line, which might be too much for any of the Accutronics pans. It is entirely possible that running a triode-strapped pentode in the driver section is a mistake, and I may have to step back down to pentode operation.

    6bm8singletubereverb2a.png

    P.S. I'm slomping through the circuit theory chapters in the ol' Radiotron Designer's Handbook, 4th. Ed. Sooner or later, I'll figure this out. But man, is it PAINFUL...
    Last edited by dchang0; 11-17-2010 at 11:30 PM.

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    Well, I'm reading the Rod Elliot article referenced by The Valve Wizard's article.

    In it, Rod says "the value of R7 (shown in Fig. 5) must be selected based on the coil impedance." Later on, he says "R7 is based on an estimation, where the resistor value is roughly 20 times the coil's 1kHz impedance. Reduce the value of R7 for less treble response and vice versa." Then, he says, " This simple circuit (Fig. 6) has a deliberately limited output impedance.... This is the equivalent of using a resistance in parallel with the coil (R7) as shown in Figure 4-all drive circuits require a high frequency limit."

    But all of this theory is explained via solid state circuits such as the one in Rod's Fig. 5. Translating this over to valve circuits is waaaay beyond my skills (or interest).

    I think I'll guesstimate my way through this. Start with the 8F tank with its high impedance, cut down the RMS voltage and current of the driver output to where it's close to the safe specs of the tank at full saturation, and call it a day. (Forgetting about optimum impedance matching and tone...)

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    Sent an email to Merlin, the Valve Wizard himself, to see if he can help me through the example he gave on the paralleled ECC82s.

    It's pretty much all I need to know to pull this off.

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    Taking another approach. I am reverse-engineering the Ampeg Reverberocket II and basically trying to match the performance of the 6BM8 t.s.p to that of the 6U10 triode. They are capable of producing similar output currents. The 6U10 as configured is set for a load line of 34mA at 340V. My 6BM8 is currently set for 55mA at 275V, and that line is on the hot side. Let's try it with a 10K Ra resistor (same as the Ampeg) to bring the load line down to 27.5mA at 275V. The Ampeg appears to be biased at -9V and at quiescent current of 9mA and quiescent voltage of 250V. This is a slightly colder biased tube.

    This raises a choice: run the 6BM8 pentode as a pentode or as a triode? The pentode is capable of producing about the same current range as the 6U10, and the t.s.p can product roughly 20mA more. In pentode mode, the curves are far more linear, but the bias voltage range is much wider on the t.s.p (0 to -38V vs. the pentode's 0 to -10V). The 6U10's bias voltage range is 0 to -24. That means the t.s.p 6BM8 is probably a better fit.

    Middle of the road bias point of -14V as before / 13.5mA = 1K cathode resistor. That's the same as the Ampeg

    Cathode cap:
    Ck = 1/(2*pi*Rk*f-half-boost-k) = 1/(2*pi*1k*5k) = 1/31.4 = 0.0318 ~= 33uF. Xicon makes a cheap 450V model. The Ampeg uses a 10uF instead. They must prefer a different roll-off frequency than 5kHz. I'll keep my 33uF.

    Ra = 10k, ra = 8V / 3.7mA = 2.162K. Best load = 4.25K.

    Power rating is actually 1.8W, so we could use a 2W or 3W for Ra. Use 3W to be safe.

    RI = 7.5K/2W. Ampeg does not use an RI or the "constant current" set up as shown in The Valve Wizard's example. I could copy either Ampeg or the Valve Wizard here. Might as well copy Ampeg to save components and also gain this huge gift: I now know that I can use the 8F 1925-ohm high-impedance tank!

    Can't seem to find a US supplier of these online yet. Ampeg uses the 1475-ohm 4F tank; I *could* go down to an 8E tank at 800 ohms and just stick a 680 ohm resistor in front. Close enough without getting into the gory details of impedance vs. inductance.


    Okay, here's the new schem, Rev. 2.0b. Ampeg-style driver, but using a triode-strapped-pentode 6BM8. Same recovery section as before. (I will double-check the gain produced in the Ampeg to make sure it's about the same.)

    6bm8singletubereverb2b.png

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    Well, this is interesting. Amplification factor mu = 55 for the 6BM8 recovery triode and mu = 92 for the 6U10 recovery triode (roughly 1/2).

    And for the driver sides: mu = 6.25 for the t.s.p. 6BM8 and mu = 17.5 for the 6U10 driver triode.

    So the Ampeg circuit has quite a bit more voltage amplification than the 6BM8 setup, with the output currents set up to be similar to one another. I could screw around with the load lines some more to get a better compromise, balancing voltage and current similarities...

    There's also the matter of the 8F tank being hard to find and also being different than the 4F tank's impedance.

    What to do?

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    Awwww, yeah!

    It hit me that if I chose the 8E tank instead of the 8F, I'm dropping the impedance from 1475 to 800 ohms. Oversimplifying the driver circuit into V=IR, if V is constant, then I = I0 * 1475/800 = 13.5mA * 1.84 = 24.84mA. The 5K Ra setup I had before was producing about that much quiescent current. AND, because the curves are better in that area, mu = 14, which is much closer to the 17.5 of the 6U10.

    So, let's go back to the original Ra=5K design on the driver side.

    On the recovery side, we'll keep the Barbour-inspired design (which is middle-of-the-road, most clean headroom, least clipping). It may not have as much gain as the Ampeg recovery, but we can probably control that using the mix resistor. And I don't necessarily want the Ampeg sound, just the Ampeg-style capacitor coupling directly to the reverb tank.

    Here is the new schem. 2.0c. This is what I'll build, barring any errors.

    6bm8singletubereverb2c.png
    Last edited by dchang0; 11-18-2010 at 12:05 PM.

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    I don't think that you'll have a problem with the lower gain on the recovery triode. I'm just completing a single tube reverb with a 12DW7, and for the recovery I've had to reduce the gain down to ~ 30 to keep the reverb from getting out of hand. As you said, you can just tailor the mix resistor to get the balance that you want.

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    Thanks, Jimboyogi--I'll leave it as is, then.

    Time to order parts!

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    Obviously I couldn't help out as much as I would have liked too-- I think you're far beyond me in terms of understanding. One question I had was your calculation of the optimum load for the triode strapped 6BM8. How did you derive that? I've always looked at the internal resistance of the tube-- which is 20k for the pentode but obviously this is much lower in a triode.
    As well, I found this schematic yesterday which uses the ECL82/6BM8 as a current source driver. It also uses a second tube the 12AX7/ECC83- which is something you may or may not have to look into. If I remember your schematic correctly you were driving the t.s.p. and a second stage from the plate of the first 12AX7 triode?
    Attached Files Attached Files

  31. #31
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    Just curious, what bright cap are you going to use in the reverb feed?
    In this forum everyone is entitled to my opinion.

  32. #32
    Noodle of Reality Steve Conner's Avatar
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    Quote Originally Posted by dchang0 View Post
    I'd love to keep my Princeton Reverb clone (sounds amazing), but it doesn't fit into our modular shelving like a Champ does.
    I've carefully read through this thread, and I think the overall best solution might be a new GF...
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  33. #33
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    Quote Originally Posted by Steve Conner View Post
    I've carefully read through this thread, and I think the overall best solution might be a new GF...
    +1 on that conclusion.

    The reverb pot (on the return side) needs to be wired with the signal going into the wiper, 1 end of the pot grounded, and the other end of the pot connected back into your circuit. Wiring it like a normal volume pot will have unintended consequences for the entire signal when you turn the reverb pot down.

    Search here for tubenit's 1-tube reverb circuit. It works. Using a 12DW7 instead of an 12AX7 is an interesting idea, particularly with only one triode ahead of the reverb driver (pentode or triode).

    Here's an example with an SCH file you can play with: Champ (&5F2 Princeton) w/reverb variations

    Cheers,

    Chip

  34. #34
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    Hey, you helped quite a bit, BiBi.

    Optimum load came from both the Valve Wizard and the Radiotron Designer's Handbook, although they same somewhat conflicting things. VW says that for maximum power, the load should be 2 x ra (anode resistance). RDH says that it should be AT LEAST 2 x ra. So, we merely need to calculate ra, which changes based on the chosen bias point. It's too long to explain here, but The Valve Wizard has it on page 22 of Chapter 1 "The Common Gain Stage," which he gives away for free on his website in PDF form.

    When you see 20k as the internal resistance of the tube, you are probably looking at the MAXIMUM or AVERAGE value, not the value calculated for your chosen bias point. There's nothing wrong with using the max or average. I only bothered to go through the calculations because I wanted to re-learn the math behind it all. It was also nice to be able to choose a bias point that I like (cleanest, least distortion) rather than say, what someone else likes (more crunch, more whatever).

    Hey, thanks for the schem! I'll use that to double-check what I've drawn up.

    My plan is:

    guitar -> 12ax7 triode (Fender 5F1 values) -> 0.022uF coupling cap -> reverb (470k bridge) ->master volume pot -> 12ax7 triode -> 5F1 power section

    Hmm. It may be time to start drawing up the new, complete schematic for the 5F1 with the reverb inserted...


    Edit: DOUBLE THANKS for the 500 R2 schematic. I just noticed that there is NO resistor used in the triode strapping. Though that is inadvisable, if the tube isn't being driven near any cutoff, it could be a way to save some space on the eyelet board layout, if I need the space.

    Unfortunately, Tom Schlangen, the guy who compiled the curves for the t.s.p 6BM8 didn't specify the way in which he strapped the pentode. I think I might reverse-engineer the values on the 500 R2 schem to see what the designer did there.
    Last edited by dchang0; 11-20-2010 at 12:37 AM.

  35. #35
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    I don't know yet. Thought I might start with 500pF and experiment from there.... At this point, I'm so burnt out on calculations that I don't want to go back in and figure out which is the "best" value...
    Last edited by dchang0; 11-20-2010 at 01:03 AM.

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